1. Field of the Invention
This invention relates to shrouded turbine blades and more specifically to the mass reduction of shrouded turbine blades while not compromising the resistance to shroud bending stresses.
2. Description of Related Art
Gas turbine blades are rotating airfoil shaped components in series of stages designed to convert thermal energy from a combustor into mechanical work of turning a rotor. Performance of a turbine can be enhanced by sealing the outer edge of the blade tip to prevent combustion gases from escaping from the flowpath to the gaps between the blade tip and outer casing. A common manner for sealing the gap between the turbine blade tips and the turbine casing is through blade tip shrouds. Not only do shrouds enhance turbine performance, but they serve as a vibration damper, especially for larger, in radial length, turbine blades. The shroud acts as a mechanism to raise the blade natural frequency and in turn minimizes failures due to extended resonance time of the blade at a natural frequency. A portion of a typical turbine blade with a shroud is shown in FIG. 1. The figure shows turbine blade 10 with an airfoil section 11 and shroud 12. The shroud is manufactured integral to the airfoil 11. The airfoil further contains a leading edge 15 and trailing edge 16 that run generally perpendicular to shroud 12. Shroud 12 has a thickness and has sidewalls 17, which are cut to create an interlocking configuration when adjacent turbine blades are present. The interlocking mechanism occurs along two bearing faces 13. That is, along bearing face 13 is where adjacent turbine blades (not shown) contact shroud 12. It is the interlocking of the turbine blade shrouds 12 at bearing faces 13, that creates the means for damping out vibrations as well as for sealing the hot combustion gases within the turbine gas-path. An additional feature of a typical turbine blade shroud is knife edge 14. Depending upon the size of the blade shroud, one or more knife edges may be utilized. These seals run parallel to each other, typically perpendicular to the engine axis, and extend outward from shroud 12. The purpose of these seals is to engage the shroud blocks of the turbine casing (not shown) to further minimize any leakage around the blade tip. While the purpose of the shroud is to seal the combustion gases within the flow path as well as to provide a means to dampen vibrations, the shroud has its disadvantages as well.
A drawback to the shroud concept is the weight the shroud adds to the turbine blade. During operation, the turbine blades spin on a disk, about the engine axis. A typical industrial application includes disk speeds up to 3600 revolutions per minute. The blades are held in the disk by an interlocking cut-out between the blade root and the disk. As the turbine blade spins, the centrifugal forces cause the blade to load outward on the turbine disk at this attachment point. The amount of loading on the disk and hence the blade root, which holds the blade in the disk, is a function of the blade weight. That is, the heavier the blade, the more load and stresses are found on the interface between the blade root and disk, for a given revolutions per minute. Excessive loading on the blade root and disk can reduce the overall life of each component.
Another drawback to shrouds is creep curling of the blade shrouds. Depending on the thickness of the shroud, the shroud edges can xe2x80x9ccurlxe2x80x9d up at their ends and introduce severe bending stresses in the fillets between the shroud and blade tip. Shrouds curl due to the bending load on the edges of the shroud from gas pressure loads as well as centrifugal loads. The curling of a shroud is analogous to the bending of a cantilevered beam due to a load at the free end of the beam. An industry known fix to this curling phenomenon is to increase the section thickness of the shroud uniformly which will result in a stiffer shroud and more resistance to curling. The downside to simply increasing the shroud thickness uniformly is the additional weight that is added to the shroud by this additional material.
It is an object of the present invention to provide an improved turbine blade shroud that reduces the overall blade mass, which in turn reduces the amount of pull on the turbine disk, increasing the life of both the turbine blade root and corresponding disk locations.
It is a further object of the present invention to provide an improved turbine blade shroud that does not compromise shroud bending stresses or mass balance of the shroud.
It is yet another object of the present invention to provide an improved turbine blade shroud that includes a rib section along the shroud for drilling cooling holes such that no high stress concentrations occur from these cooling holes.